their closure the other parts of the mouth, which are very beautifully constructed for the purpose, are brought into fitting mechanical positions for completing the work of emptying the victim. Nagel states that the larva of _Dytiscus_ injects a digestive fluid into the body of its victim, and that this fluid rapidly dissolves all the more solid parts of the prey, so that the rapacious larva can easily absorb all its victim except the insoluble outer skin. The abdomen consists of only eight segments, and a pair of terminal processes; the stigmata are all more or less completely obsolete—according to species—with the exception of the pair on the eighth segment at the tip of the body; the terminal segments are frequently fringed with hairs, that serve not only as means of locomotion, but also to float the pair of active stigmata at the surface when the creature rises to get air. Although the larvae of Dytiscidae are but little known, yet considerable diversity has already been found. Those of _Hyphydrus_ and some species of _Hydroporus_ have the front of the head produced into a horn, which is touched by the tips of the mandibles. Dytiscidae are peculiar inasmuch as they appear to flourish {213}best in the cooler waters of the earth. Lapland is one of the parts of Europe richest in Dytiscidae, and the profusion of species in the tropics compared with those of Europe is not nearly so great as it is in the case of most of the other families of Coleoptera. About 1800 species are at present known, and we have rather more than 100 species in Britain.[94] SERIES III. POLYMORPHA. _Antennae frequently either thicker at the tip (clavicorn) or serrate along their inner edge (serricorn); but these characters, as well as the number of joints in the feet and other points, are very variable._ Upwards of fifty families are placed in this series; many of these families are of very small extent, consisting of only a few species; other families of the series are much larger, so that altogether about 40,000 species—speaking broadly, about one-fourth of the Coleoptera—are included in the series. We have already (p. 189) alluded to the fact that it is formed by certain conventional series, Clavicornia, Serricornia, etc. united, because it has hitherto proved impossible to define them.